Electrical control circuits



29, 1935. o. w. LIVINGSTON 1 ELECTRICAL CONTROL CIRCUITS Filed March 29, 1934 2 Sheets-Sheet l Fi gl. 5

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Inventor: OTTiTI W Livin ston.

Attorne Oct. 29, 1935.

O. W. LIVINGSTON ELECTRICAL CONTROL CIRCUITS Filed March 29, 1934 2 Sheets-Sheet 2 F Fig. 5.

Inventor:

Orrin WLivin ston,

His Attorne Patented oct- 29, 1935 UNITED STATES PATENT OFFICE ELECTRICAL CONTROL CIRCUITS Orrin W. Livingston, Scheneetady,-N. Y., assignor to General Electric Company, a corporation of New York Application March 29, 1934, Serial No. 718,094

' 17 Claims. (01. 175-4 3) 5 ra lity of related electrical quantities to respective maximum values. Furthermore, it is often deslrableto regulate one of said quantities to a constant. value so long as the others are below their respective maximum values and successively to transfer control as the other quantities reach their maximum values, so as to limit them to said values. As a two quantity example, it is often desirable .tomalntain the voltage of an electric translating circuit constant over a given current range, while at the same time limiting maxi mum current in the circuit to a predetermined value.

In accordance with my invention, I provide a novel vacuum tube control circuit for accomplish- .ing the results outlined above. Briefly stated, this control circuit operates on the principle that if variable potentials, corresponding to the two quantities mentioned above, are applied, respectively,-'to vacuum tube anode-cathode conducting paths and if one of the corresponding electrodes of each path .is interconnected so as to have the same potential, current will tend to flow only between the anode and cathode of the path to which the greater potential is applied. This is particularly true for small current values, such .as can be employed for controlling the grids, or control .electrodes, of vacuum tubes. The explanation of this is that for such small currents, the voltage drop in an anode-cathode conducting path is so small as to be negligible, so that for example, if the corresponding electrodes are the cathodes of the paths and if positive potentials are applied to the anodes .of the paths, the cathodes will automatically become more positive than the anodes of all of the paths except the path to which the highest anode potential is applied. As my invention utilizes a plurality of anode-cathode paths each having an equi-potential corresponding electrode, I may utilize a single multi-electrode vacuum tube having a common electrode, such for example, as a common anode with respect to a plurality of cathodes. or a common cathode with respect to a plurality of anodes. With such a device it is unnecessary to provide an interconnection becommon electrodes, as the common electrodes are one and the same element. As

applied in my invention I refer to such a device as a transfer tube. I

It is immaterial to my invention in its broader aspects whether the anode-cathode conducting paths of my invention are in high vacuum pure electron discharge type vacuum tubes, or wheth er they are in, gas or vapor filled arc discharge tubes. I at present prefer to employ a high vacuum type' tube because at the small values of effect of such tubes is negligible, so that the voltage drop therein is very small and highly uniform. In the present commercial are discharge type tubes the arc drop is sometimes erratic, and this tends to interfere with the reliability of operation of my invention.

One practical application of my invention is to storage battery charging circuits wherein my regulating system may be made to hold constant voltage on the charging circuit up to a predetermined maximum current value. Another application is to testing vacuum tubes.

An object of my invention is to provide new and improved electrical control circuits.

. Another object of my invention is to provide new and improved vacuum tube control circuits.

My invention will be better understood from the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings, Fig. 1 is a diagrammatic illustration of an embodiment of my invention employing'a double anode transfer tube, Fig. 2 shows the voltage-current characteristic of the circuit shown in Fig. 1, Fig. 3 is a modified and simplified circuit showing the use of a double cathode type tube, Fig. 4 illustrates a modification of Fig. 3 wherein two double electrode tubes 3 having interconnected anodes may be substituted for the double cathode transfer tube of Fig. 3, and Fig. 5 illustrates a modification of Fig. 1 wherein two double electrode tubes having interconnected cathodes may be substituted for the 4 double anode tube of Fig. 1.

In Fig. 1 there is shown an electric translating circuit comprising 'n alternating current supply circuit I for energ g a rectifier 2 whose output energizes a direct current load circuit 3. Recti- 4 fier 2 is shown by way of example as a bridge connected copper oxide type rectifier, but it should be understood that my invention is not limited to any particular type of rectifier and that for applications of my invention to trans- 5 lating circuits handling appreciable current, it would probably be preferable to use vacuum tube rectifiers of the arc discharge type, such as shown in Fig. 3. In the alternating current supply circuit I there is connected a control, or regulating device 01' any suitable type, such as a saturable as a grid controlled rectifier tube. This tube may be of any of the several types well known in the art, but I prefer to use a vapor electric discharge type controlled rectifier because of its greater current carrying capacity and efllciency at higher current values. This tube operates in a well known manner to rectify the output of transformer 6 whenever its grid potential is above a predetermined critical potential with respect to its cathode and to block the current flow between anode and cathode, and consequently the rectifying action, whenever the grid potential is below this predetermined critical value.

The circuit of Fig. 1 controls the conductivity of rectifier tube I in response to both the current and voltage of the translating circuit by means of the following arrangement, the main element of which is a double anode vacuum tube 8 which serves the purpose of automatically transferring the control from voltage to current at a predetermined maximum current value. Tube 8 may be of any of the several well known types of double anoderectifier tubes, such for example, as a hot cathode bi-phase high vacuum pure electron discharge type rectifying tube. A circuit 9 is arranged to apply a positive potential to a the right-hand anode of tube 8, with respect to its cathode, which is proportional to the voltage of circuit 3. As shown, this circuit connects the positive and negative sidw of the circuit 3 to the right-hand anode and the cathode of the tube 8, respectively. Any suitable means for adjusting the relation between the voltage of circuit 3 and the voltage applied to the right-hand anode of tube 8 may be added to the circuit 9 and as shown, a suitable means of this type is an adjustable potentiometer rheostat Ill connected in circuit 9. In a similar manner, a circuit II is arranged to apply a positive potential to the left-hand anode of tube 8, with respect to its cathode, which is proportional to the currentflow in the translating circuit. As an example of such a circuit there is shown a current transformer l2 connected in the supply circuit i and arranged to energize a suitable rectifier l3 whose output is connected with the proper polarity between the cathode and left-hand anode of tube 8. The circuit II is also provided with suitable adjusting means, such as a shunt rheostat ll, for varying the relaflon between the gnitude of the current in circuit I and the posi ive potential applied to the left-hand anode of the tube 8. Connected in circuit with the cathode of tube 8 is a resistor IS, the potential drop across which is proportional to the current flow in the tube 8.

In order to provide a suitable standard or reference quantity against which my regulating system can regulate, I utilize a glow discharge tube l6 which, as is well known to those skilled in the art, has the property of exhibiting a substantially constant potential drop over a relatively wide range of cin-rent flow therethrough. The source of current supply for producing the current flow in the discharge tube l6 may be obtained from any suitable source and as shown by way of example, tube It is connected to be energized from the alternating current supply circuit I through a transformer ll, of suitable voltage ratio, and a suitable rectifier ill for producing a unidirectional potential across the ter- 5 minals of the tube I6. As an added means for insuring the constancy of the drop across the tube I6 I utilize a ballast tube, or resistor [9 con-- nected in the energizing circuit for the tube l5. Ballast tubes have the well known property of 10 changing their internal resistance over a relatively wide range with very slight current changes so that they are substantially constant current devices. By the use of the tube IS the range of current flow through the glow discharge tube l6 15 for a given range of voltage variation on supply circuit l is greatly reduced thereby insuring that tube IE will operate at all times within the current range over which it exhibits the constant potential drop characteristic. 2(

By means of a circuit 20 glow discharge tube It and resistor ii are connected in series and the energization of the discharge tube I6 is such that the potential drop across it is in opposition to the potential drop across the resistor IS in the 25 series circuit 20. Consequently, when the current flow through the tube 8 is such that the drop across resistor 15 is equal to the drop across discharge tube It there is no net voltage in the series circuit including these devices, while if the potential drop across resistor I5 is less than or greater than the constant drop across the tube IS a reversible polarity difl'erential control voltage is obtained.

By reason of the particular circuit which I employ it is desirable to provide means for amplifying the above mentioned control voltage and also for reversing it in order. to get the proper control of the tube I for producing the desired regulatory action. This amplifying and revers- 4 ing operation is achieved by means of a Wheatstone bridge circuit including three resistors 21 as three of the arms of the bridge and a controllable electric valve 22 as the other arm thereof. This bridge may be energized in any suitable 4 manner and as a convenient way of obtaining its energization it is connected with its input terminals in circuit with the glow discharge tube It so as to be energized from the same source of current supply as the tube l6. Valve 22 may be 5 of any suitable iype, such for example as a high vacuum pure electron discharge type amplifying tube. This tube is so connected in the bridge circuit that its cathode is connected to the negative input terminal of the bridge and its anode is con- 5 nected to the right-hand resistor 2|. The circuit 20 containing the differential voltage between the constant voltage drop in the tube l6 and the variable voltage drop in the resistor I 5 is con- .nected between the grid and cathode of the tube 6 22 in such a way that whenever the drop in the resistor I5 is greater than the constant drop in the tube [6 the potential of the grid of tube 22 is positive with respect to its cathode, while when the drop in the resistor i5 is les than the drop 6 in the tube It the potential of the grid of tube 22 is negative with respect to its cathode. The grid control circuit of tube 1 is connected across the output terminals of the bridge in,such a manner that when the grid of tube 22 ls -negative 7 with respect to its cathode, that is to; say, when the impedance of tube 22 is high, the potential of the grid of tube I will be positive with respect to its cathode, while when the potential of the grid of tube 22 is positive the potential of the time at will be ma ash its cathode. '11.; :1 t n" "2 A In order to secure as pure direct current 'co'n-' -one,'which .are electrically adiaeent tube 22 in the bridge circuit and arrange this impedance to, have a negative impedance current characteristic.- One such impedance which .I have found to be satisfactory is thelwell known lightning ar+ restor material, known to the trade as Thyrite, whose composition is described and claimed in Patent NO."1,822,742, issued September '8, 1931, on an application of Karl B. McEachron, and as signed to the assignee of the present application;

This material has the property ofireducing its electrical resistance with increases in voltage ap-' .plied theretoand as thefllarnent-plate characteristic of a high vacuum tuba-such as tube 22, is such that its .resistance'decreases with increases in voltage, the characteristics of the tube 22 and of the impedance 26 compensate each other in .the bridgecircuit so that the output potential of v the bridge which is applied to the grid of the rectifier I is, substantially independent of the output voltage of the rectifier l8.

It is preferable .to control the conducivity of .rectifler 1 by shifting-the phase of itsgrid potential rather than by straight-magnitude control of its grid potential. This is 'becausephase shift control is a more continuous control than magnitude control because of the fact that control takes place within the half cycles of positive anode potential ratherithan during successive half cycles of said anode potential in which "the, tube I is .either nonconducting. or .wholly' conducting throughout-a complete half cycle. One simple way of securing. phase-shift control is to insert in the grid-cathode circuit of :ihe rectifier I a relatively small alternating current potential which lags the potential of the anode-cathode circuit by 90 degrees. The ,means which I have shown for inserting this quadrature lagging alternating current component consists of a well known impedance phase shifting circuit comprising a resistor 21 and a condenser .28 connected across a mid- ,tapped reactor -which inturn is connected across the alternating current supply circuit I. To the midpoint of the reactor 29 and-to the point between the resistor 21 and the condenser 28 a potential transformer "is connected with its secondary winding in thegrid circuit of the rectifler I; By suitably choosing the values of the reslstor2'l and condenser 28 thevoltage drop therein maybe so proportioned that the potential of the point between them is, with respect to the potential of the'midpoint of the reactor '29, in quadrature "with the supply. circuit voltage and by suitably "connecting the transformer this potential may be made la ging in the grid circuit ofthe tube 1.

' It should of course be understood that'Fig. 1

is merely a diagrammatic andsimplified showrent for the reactor l. Ina'commercial'circuitit would ordinarily be preferable to utilize a full ,ative with respect to its cathode. This substantially reduces the current conducting value of min-'- ciple'is' the {same it is believed to further complicate Fig. 1 by showing such a full wave rectifier.

wave rectifier purpose, but the "The operation of Fig. 1 results a regulated 5 characteristic which is substantially rectangular in shape and which is illustrated by the ideal regulator characteristic shown in Fig. 2. In this figure, the voltage of the load circuit 3 is shown constant at afpredetermined value, designated as 100%. ,up to a particular predetermined maximum current value, also designated as 100% whereupon the current is held constant and is limited to this 100% value from 100% to zero voltage.

Ignoring for, thetime being the action ofthe phase shifting means, the way the circuit of Fig. 1 operates to} produce this regulating character-- istic is as follows. Assume that alternating-current supply circuit I is energized by any suitable source of current supply (not shown) and that a load is connected to the direct. current circuit load} and that the value of this load is such that the current is less than the 100% current shown in' Fig. 2. Under these circumstances, ,-poten-.25 tiometer I 0 is adjusted so that with 100% voltage on circuit the potential drop across the re sistor l5 produced by the current from the righthand'anode of tube 8 to the cathode is just equal tofthe constantvoltage drop in the glow discharge tube It. Also the rheostat I4 is adjusted so that for,100% current flow in the translating circuit, the potential of the left-hand anodeoi tube 8 with respect to the cathode, will just equal the potential of the right-hand anode of the tube with respect to the cathode with 100% voltage on circuit 3. However, as it has been assumed that for the present the qirrent in the translating circuit is less than the 100% value, the potential oi the right-hand anode will be greater than the 0 potential of the left-hand anode of the tube 8, and aslcurrent only flows between the cathode and the anode at high potential, as previously explained, it will be seen that the potential drop across the resistor IE will be proportional to the voltage of circuit 3. Assume now that the voltage in circuit 3 dropsslightly below the 100% value. As has been explained above, this will make the grid of tube 22 negative. This greatly increases the impedance of the tube 22 with the result that the bridge circuit is unbalanced in such a direction that the point to which the grid of tube 1 is connected becomes positive with respect to the glow tube It, thereby makingthe grid of tube 22 5 positive. This causes the tube to become conductive and greatly reduces its impedance value therebyv unbalancing the bridge in the opposite direction and making the grid of the tube 1 negtube 1 to zero thereby preventing a current flow .in the saturatingwinding 5 of the reactor whereby its impedance is greatly increased, thereby increasing the voltage drop across it and reducing the voltage of circuit 3. As a result, the poten tial drop across the resistor] 5 oscillates at a high rate speed,' 'which depends upon thelconstants of the entir'e' circuit, 'between a value S gh y above "and a value slightly below the constant drop across the glow tube It with the result that the voltageof circuit 3 is held substantially at the 100% value at which it is desired to hold it.

' The above described "operation is analogous to the operation 01" a Tirrlll voltage regulator, in that the-action is-intermittent because the rectiiier l is either wholly conductive or wholly nonconductive during the hall cycles of posit ve anode potential, thereby tendingto decrease or increase the voltageiot circuit 3 by a amoimt whereby the average voltage held is'the normal voltage. Howeveathepresenceoi' the phase shifting circuit produces a much smoother action than the -Tirrill action described above because the quadrature lagging alternating current ripple which is superposed on the direct current grid potential or the tube 1 produces a grid potential for the tube I which has a sloping wave front so that control "can be secured by vary I small changes in the amount or direct current'component of the grid voltage of the tube]. For example, at normal voltage of circuit 3, the current through tube 22 is just sufficient to produce a direct current component in the grid circuit of tube 1 v which, when added to the alternating current component, will produce a control potential which causes the tube 1 tobecome conductive in the middle of its halt cycle of positive anode 130-: tential. Consequently, the tube 1 is conductive for the latter hall of each successive hall cycle of positive anode potential, thereby holding a substantially uniform average current in the saturating winding 5 of the reactor 4. If now the voltage circuit 3 falls slightly, the direct current component, or output otthe bridge circuit increases slightly, thereby causing the'total grid voltage of the tube 1 to be raised, and due to the alternating current component, this means that the tube 1 becomes conductive earlier in the hall cycles of positive anode potential, thereby increasing the average directcurrent in winding 5 and increasing the saturation of reactor I whereby the voltage of circuit 3 is increased. Similarly, if the voltage of circuit 3 increases above normal the output voltage oi the Wheatstone bridge'decreases with the result that the tube 1 becomes conductive later in each half cycle of positive anode potential, therebyreducing the average current in the saturating winding 5 and increasing the impedance of the reactor, whereby the voltage of circuit 3 is reduced. This results in a substantially continuous control whereby the time of conductivity of the tube 1 during each,

half cycle. of positive anode potential bears a definite relationship to the voltage of circuit 3.

It now the current ilow in the translating circuit tends to exceed the predetermined 100% maximum value, the potential of the left-hand anode of tube exceeds the potential or the righthand anode and this has the eflect of automaflmlly transferring the control Irom voltage to current, because all of the current in the tube i will now flow from the left-hand anode instead of from the ht-hand anode so that the voltage drop across resistor I! will now be proportional to current instead oi to voltage. Consequently, as soimasthecurrent exceeds the 100% value set by the rheostat I, the grid of tube 32 becomes podtive thereby rendering the grid of tube 1 negativeandstoppingtheflowofcurrentinthe amazes saturaflng winding 5 or I the reactor whereby the impedance oi thereactoris increased,z-thereby increasing its current limiting ability and decreasingthe current in, the load circuit. Consequently, the current cannot substantiallyzexceed the 6 100%value which hasbeen set..-

The-modification shown-in Fig. 8 differs from Fig. 1 in-that a double cathode-type transfer tube 3| is used-and thediflerential control voltage oi circuit is applied directly tocontrolling'lfl the rectifier 1 instead of interposing a-v'alve con trolled bridge, as in Fig. 1. Also, for the sake of simplicity, the various filters and the phase shifting circuit'ot Fig. l have been omitted. Another minor diil'erence prises a bi-phase rectifier consisting of an autotranstormer 32 and rectifier tubes 33 in place '0! the copper oxide rectifier! of Fig.1. 1

As shown the transfer-tube 3| has two'anodes which are interconnected, but it is obvious that 20 but one anode could be employed if dwired, as the two interconnected anodes are the equivalent oi. but a single anode. Similar-1min Fig. 1, two interconnected cathodes could-have been employed instead of the single cathode, if desired. Such S8 changes are more or less arbitrary 811111111 no sense change the principle of operation.

Circuits 8 and .l I are connected to the anodecathode circuits of the tube 3 I insuch a manner thatthe normal voltage drop in the resistance II 80 is reversed, and similarly. the rectifier 18 is so 1 connectedthat the normal resistance drop in the glow discharge device 16 is reversed with-respect" to these drops in Fig.l.- f "I" The operation of Fig. 3 isas follows: It the 36 voltage or circuit 3 rises above normal'the grid of tube 1 becomes negative with respect to its cathode thereby interrupting the iiow oi current through the tube I and increasing the impedance at reactor 4 thereby decreasing the voltage oi! 40 circuit 3. n the voltage of circuit -3 runs below normal the constant drop across glow tube l6 exceeds the drop across resistor l5 thereby making the grid of tube! positive whereby the tube becomes conductive; thereby causing saturating (5 current to flow in the winding Ii whereby the re actor 4 becomes saturated thereby reducing its reactanceand increasing the voltage of circuit 3. This action continues until the current incircuit 3 tends to exceed the 100% value whereupon 5d the potential of circuit u exceeds the potential of circuit 8 whereby current flows fromthe anode oi tube ii to its left-hand cathode and as the drop across this path is small the anode becomes negative with respect to the right-hand cathode so that control is secured by the circuit ll entirely. Consequently, as the current increases above 100% the grid of tube '1 becomes negative thereby stopping the flow of saturating current in the winding 5 and increasin th 1 to pedance oi the reactor 4 whereby the current in circuit 3 is substantially limited to the 100% value. I

It will, of course. be obvious to those skilled in the art that ii desired the filters and phase shitting circuit of Fig. 1 can be added to Fig. 3 without altering the essential principle oi operation but merely producing the various specific im-' provcments noted in connection with Fig. 1.

Fig. 4 illustrates a pair or double ectrode tubes 34 and 35 connected in a manner in which they could be employed in Fig. 3. Thus, the interconnected anodes could be connected to the a resistor i5 and the cathodes of tubes and II 7 is that the main rectifier com- 15 v could be connected respectively to the circuits Q and H.

In Fig. are shown a similar pair 01 tubes connected in a manner in which they could be substituted for the tube I of Fig. 1. When so substituted. the common cathodes will be connected to the resistor l5 and the anodes oi tubes 84 and 35 will be connected respectively to the control circuits 9 and H.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that changes and modiflcations may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifictions as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, means for producing a plurality of relatively variable control potentials which are responsive respectively to separate quantities which are to be selectively regulated, electroresponsive means, and vacuum tube means for selectively causing said electroresponsive means to respond only to the greater of said potentials.

2. In combination, apparatus having a pair of relatively variable quantities which are to be selectively regulated, means for producing a pair of unidirectional control potentials which vary respectively in accordance with the values of said quantities. control means for said quantities, and vacuum tube means for selectively causing said control means to respond only to the greater of said potentials.

3. In combination, apparatus having a plurality of variable quantities which are to be selectlvely regulated, means having a plurality of anode-cathode conducting paths, means for maintaining a corresponding electrode of each path at substantially the same potential, means for applying to each anode-cathode path, respectively, a potential which varies in accordance with a difterent one of said quantities, and electro-responsive means connected to be responsive to the anode-cathode current through said corresponding electrodes.

4. In combination, apparatus having a plurality of relatively variable quantities which are to be selectively regulated, means having a plurality of high vacuum pure electron discharge anodecathode conducting paths with an efiective electrically common electrode, means for applying to each of said paths respectively a potential which varies in accordance with a different one of said quantities, and electroresponsive means responsive to the common electrode current.

5. In combination, apparatus producing a plurality of variable quantities, control means for said quantities, said control means comprising a plurality of anode-cathode vacuum tube conducting paths having means for maintaining a corresponding electrode of each path at the same potential, means for applying to each anodecathode path, respectively, a direct current potential which varies in accordance with the value of a difierent one of said quantities, the positive side of said potential being applied to the anodes of said paths, and means responsive to the anode-cathode current through said corresponding electrodes for operating said control means.

6. In combination, an electric valve, means having a plurality of differently controlled anode-cathode conducting paths with an effective electrically common electrode, means for impressing relatively variable potentials on said paths, and means for controlling the conductivity of said valve in accordance with the current 5 flow through said common electrode.

7. In combination, an electric circuit, a saturable reactor therein, a grid controlled rectifler for controlling the value of saturating current in said reactor, means for producing a substantially constant control potential, means including a vacuum tube for producing a control potential which is variable in accordance with the voltage of said circuit up to a predetermined value of current in said circuit and which is variable in accordance with said current at values thereof above said predetermined value, and means for controlling the conductivity 01' said rectifier in accordance with the diilerencc between said potentials.

8. In a vacuum tube control circuit, means for producing a pair of relatively variable umdirectional control potentials either one or which may exceed the other, means responsive to the greater only of said potentials comprising a double an- 2 ode vacuum tube to whose anodes the positive sides of said potentials are respectively connected and to whose cathode the negative sides of said potentials are connected whereby current can only flow between the cathode and the anode at the higher potential, and means responsive to said current for producing an electrical control quantity.

9. In combination, an electric translating circuit having two variable electrical quantities, and means for limiting said quantities to respective predetermined maximum values, said means including a double anode vacuum tube to whose respective cathode-anode circuits potentials are applied which vary respectively in accordance with variations in said respective quantities.

10. In combination, an electric translating circuit having two variable electrical quantities, a double anode vacuum tube, means for applying direct current potentials which vary respective- 4L ly with variations in said quantities to the respective anode-cathode circuits of said tube in such a way that the negative sides of said potentials are connected to the cathode oi. said tube, and means responsive to the magnitude of 5D the anode-cathode current flow in said tube produced by whichever of said potentials is the greater for regulating the value of the quantity of said circuit corresponding to said higher potential.

11. In combination, an electric translating circuit, a regulating device for controlling the current and voltage of said circuit, a double anode vacuum tube, means for impressing on one of the anodes of said tube a positive potential with respect to the cathode of said tube which varies with the voltage of said circuit, means for impressing on the other anode of said tube a positive potential with respect to said cathode which r varies with current in said circuit, and means responsive to the current flow in said tube for controlling said regulating device.

12. In combination, an electric translating circuit, means for holding the voltage of said circuit substantially constant up to a predetermined value of current flow therein and for limiting said current flow substantially to said value comprising a double anode high vacuum rectifying vacuum tube, means for applying a positive potential with respect to the cathode of said tube to one of the anodes thereof, which potentialisproportionaltothevoltageotsaidcircult, means for applying a positive potential with respect to the cathode of said tube tothe other oi the anodes oi said tube, which last mentioned potential is proportionalto \the current in said circuit, and means for regulating the impedance of said circuit in accordance with'the current flow in said tube.

13. In combination, an alternating current supply circuit, a direct current load circuit, a rectiiler interconnecting said circuits. a saturable reactor in said alternating current supply circuit for controlling the current and voltage of said direct current load circuit, means including a grid controlled rectifier for controlling the saturation of said reactor. a bridge circuit including a grid controlled electric valve as one arm thereof, circuit conductors for connecting the grid circuit of said rectiiier across the output of said bridge, a double anode vacuum tube, means for impressing on one of the anodes of said tube a positive potential which varies in accordance with the voltage of said load circuit, means for impressing on the other anode oi said tube a positive potential which varies in accordance with the current in said load circuit, and means responsive to the magnitude oi" the current flow through said tube for varying the grid potential of said valve.

14. In combination, means for producing a pair of relatively variable unidirectional control potentialavacuumtubemeanshavingapalrof cathodes and an eflective electrically common anode, the positive side oi said potentials belnx connected to said common anode and the negative sidesoisaidpotentialsbeingconnectedto said cathodes respectively whereby current normallywillflowonlybetween saldcommonanode and the cathode at the lowest potential diilerenee o with respect thereto, and means responsive to said current for producing an electrical eimtrol quantity. 15. In combination, anelectrlc cir-' cult having two variable electrical quantities, and Ill means for limiting said quantities to rupective predetermined values, said means including a double cathode single anode vacumn tube to whose respective cathode-anode circuit poten-' tials are applied which vary respectively in'acl5 ace-game with variations in said respective quan- 10. In combinaticna Wheatstonebridge laving input and output terminals, a high vacuum tube connected-in one arm of said bridge. and20 means for compensating the output voltage oi said bridge against variations in voltlle applied to the input terminals thereof comprising an impedance having a negative resistance-current characteristicconnectedasapartoianarmotfi said bridge which is electrically adjacent the arm containing said tube.

17. In combination, a Wheatstone bridle. lving input and output terminals, a high vacuum tube connected as one arm of said bridsa'and 30 means for compensating the output ol'said bridge for variations in inputvoltage comprising a nonlinear impedance connected as a part of another arm of said bridge.

ORRIN W. IJVINGS'DON. l8 

